A security system utilizing a cable TV communications system is disclosed in a pending patent application entitled "Security System", by Tom O'Brien, Ser. No. 328,304 filed Dec. 7, 1981, which application is assigned to the assignee of the present invention. The latter patent application describes an alarm system utilizing three radio frequency signals as data communication channels between a headend alarm processor and a plurality of subscriber alarm processors. One channel is a polling channel on which the headend alarm processor transmits command messages to all subscriber units. Another channel is a polling response channel on which an addressed subscriber unit responds to a headend command message. The third channel is an alarm channel on which subscriber units transmit alarm messages to the headend essentially when a respective alarm condition is first detected.
Thus, a security system of the type described above has one transmitter tuned to the downstream polling channel frequency, but a plurality of transmitters respectively tuned to either the upstream polling return frequency or alarm channel frequency. If one of the plurality of subscriber alarm units should fail in a mode wherein a continuous transmission is coupled to the cable system, then one or both of the polling return channel or alarm channel will be jammed, thereby disabling the entire security system.
In a security system, continuity of security coverage is of great importance. When a system failure does occur, the system must be restored to proper operation quickly so that theres no significant interruption in security coverage.
On prior art solution to the above described jamming problem is to use a radio frequency (RF) signal sensor coupled to a switching circuit on the output of each subscriber transmitter. If the RF signal output from any subscriber unit is continuous, i.e. if rf transmission exceeds a predetermined time duration such as 10 milliseconds, then the switching circuit is rendered operative, effectively preventing the output of the subscriber's transmitter from entering the cable system.
Another prior art solution to the jamming problem is available in cable systems utilizing bridger amplifiers with bridger switches that are controllable from the headend.
Bridger amplifiers connect trunk lines to local feeder lines. Within the bridger amplifier, there is a return amplifier (i.e. a 5 to 30 MHZ amplifier which propagates signals in the reverse direction). The return amplifier includes a bridger switch that is controllable from the headend for disabling the bridger return amplifier.
When a jamming signal is detected at the headend, succesive bridger amplifier switches throughout the cable system are opened under headend control until the jamming signal is eliminated. Then all bridger switches are enabled except for the bridger switch corresponding to the feeder line containing the jamming subscriber unit.
However, a portion of the subscribers on the effected feeder line, i.e. downstream of the activated bridger switch are without security coverage. Moreover, the specific location of the jamming subscriber unit must be determined by manually checking the cable system in the field.